1. Field of the Invention
The present invention pertains to electrophotographic reproduction and, more particularly, to a method and apparatus for uniformly developing a latent charged image carried on pieces of electrophotographic paper in order to evenly distribute the lines of force of the electrostatic field of the latent charged image to eliminate the phenomenon known as the "edge effect."
2. Discussion of the Prior Art
The quality of conventional electrophotographic reproductions, whether developed by powder or liquid toner, is defective due to the phenomenon of "edge effect;" that is, the toner particles are more greatly attracted to the edges of the latent image corresponding to flat tints such that the center portion of the image is developed only faintly. The "edge effect" phenomenon is of only slight consequence in the reproduction of textual material in that the thickening of the edges of developed lines is visible normally only with a magnifying glass; however, the disadvantages of the edge effect phenomenon are extremely noticeable in the electrophotographic reproduction of images having flat tints. The disadvantages caused by the edge effect phenomenon, thus, have greatly retarded the application of electrophotostatic photography within the fields of photographic negative reproduction and reproduction of half-tones or continuous documents or negatives taken through a ruled half-toned screen.
The edge effect phenomenon is caused by an uneven distribution of the lines of force of the electrostatic fields formed by the charges deposited on the image side of electrophotographic paper even if the charges are uniformly distributed. That is, when a latent charged image is formed on electrophotographic paper, even with the charges evenly distributed over the image surface to be developed, the distribution of the lines of force of the electrostatic field formed by the charges is such that the lines of force are denser along the edges of the latent image separating uncharged areas of the paper from adjacent charged areas. Powder or liquid toners are normally composed of colored charged particles of a polarity opposite to the charged polarity of the latent image such that the particles are attracted by the electrostatic field developed by the latent image charges, and since the electrostatic fields are more intense adjacent the edges of the latent image as compared with the center of the latent image when flat tints are being reproduced, a greater quantity of toner will be deposited at the edges of the latent image causing a visibility inequality of optic density. Thus, after development, the edges of the reproduced image will be darker than the central portions.
To overcome the disadvantages caused by the edge effect phenomenon, it has been attempted to equalize or evenly distribute the lines of force of the electrostatic field and, preferably, to make the lines of force perpendicular to the plane of the latent charged image such that the resulting flux is uniform along a distribution of uniform charges thereby permitting the depositing of equal quantities of developer along such uniform charged areas. One attempt to provide such a uniform distribution of lines of force of an electrostatic field has been to immerse a conductor electrode in a developing or toner medium with the electrode connected to a specific electrical potential in order to channel the lines of force. This approach results in a slight improvement of the edge effect; however, the latent charged image on the electrophotographic paper will be discharged at the slightest contact of the electrophotographic paper with the electrode and such contact occurs all too frequently due to the requirement that the electrode be positioned extremely close to the path taken by the electrophotographic paper. To prevent such contact and discharging, it has been proposed to electrically insulate the electrode, for example, by means of nylon threads; however, due to the friction on the sensitive layer of the electrophotographic paper, the nylon threads cause streaks which distort the resulting image.
The wide use at present of processes for electrophotographic reproduction has led to the appearance on the market of a great number of development devices and particularly for the development with liquid developers.
All of the prior art devices do not give entirely satisfactory results and, in particular, French Pat. No. 2,148,835 discloses requirements that must be met by the liquid developers to cause the disappearance of the edge effect. To succeed, it is necessary to make the electrostatic field created in the liquid developer contained on conductive surfaces of fixed potential, parallel to the surface to be developed and at a suitable distance from the latter, while avoiding any contact between the conductive surfaces and the charged face of the electrophotographic paper.
This patent describes a device in which the paper to be developed is in the form of a continuous strip and passes with its back in contact with one of the electrodes, opposite and at some distance from a conductive roller which, bathing in the liquid developer and turning at a suitable speed, causes the formation of a meniscus at the surface of the passing paper. The device thus described has the drawback of making the paper pass with the photosensitive face turned downward and is not suitable for development of electrophotographic papers previously cut in sizes.
U.S. Pat. No. 3,367,791 of Lein provides a roller dipping into a liquid bath, which roller is operated at speeds such that a hydrodynamic pressure is created against the surface of the sheet, holding the latter against a back guide which is located at a fixed, determinate spacing from the closest point of roller approach. The roller is made conductive, or at least has a highly conductive surface layer, which provides the field control effect required at the point where electrostatic deposit of particles is occurring. The motion of the roller in addition to providing the hydrodynamic forces for sheet control, also renders the liquid action sufficiently turbulent to provide a constant changing and replenishing action which will forestall the possibility of the developing liquid's reaching a starved or depleted condition at the developing location where rapid particle deposit is occurring due to the presence of the highly efficient field control.
There are other prior art devices which have tried in order to make improvements which make it possible to assure sheet by sheet development and having the conductive face turned upward, however, these devices either do not assure development of such quality as to eliminate the edge effect phenomenon or else require complicated guiding means.